Activated carbon is made from carbon materials such as carbonized coconut shell, petroleum coke or coal coke that is activated to have a porous structure. The activated carbon that is porous and thus has a large surface area has been widely used as an absorbent, a catalyst support, and an electrode material for double layer capacitors and lithium secondary batteries. In particular, in order to increase the energy density, i.e., capacitance in an electric double layer capacitor, which is used in a hybrid car or the like, an activated carbon having effectively formed fine pores, a high crystallinity and a large surface area has been demanded to be used as an electrode material for the capacitor.
For industrial production of such an activated carbon with effectively formed fine pores that can be used as an electrode material of an electric double layer capacitor, an activation method has been generally used, in which a carbon material such as petroleum coke and an alkali metal compound such as potassium hydroxide are heated at a temperature of 600 to 1200° C. in an inert gas atmosphere to allow the alkali metal to ingress between and react with graphite crystal layers. In this activation, the alkali metal enters the layered structure wherein condensed polycyclic hydrocarbons are layered, as the result fine pores are formed.
The activated carbon that is used for production of an electric double layer capacitor electrode is required to have a relatively large surface area, a small average particle diameter, and a uniform particle size, and contain no bulky particles.
For example, in a process for producing the foregoing activated carbon as described in Patent Document 1, a carbon precursor such as coke is heated in the presence of oxygen so as to be oxygen-crosslinked and further calcined after addition of an inorganic salt thermal reaction auxiliary agent such as zinc chloride and a phosphoric acid salt to create micro pores thereby producing a carbon material for an electric double layer capacitor electrode. Patent document 2 proposes a process for producing a carbon material for an electric double layer capacitor electrode wherein petroleum green coke is subjected to a first calcination at a temperature range of 600 to 900° C. and then cooled to a temperature of 100° C. or lower and thereafter a second calcination at a temperature higher which is 100° C. higher than the first calcination temperature is carried out thereby producing a carbonized product, which is then activated with an alkali metal hydroxide.
However, the activated carbon for an electric double layer capacitor electrode is required to be small in average particle diameter and uniform in particle size, and contain no bulky particles. In view of such requirements, the foregoing processes are still insufficient and have been demanded to be further improved.